This invention relates to analog-to-digital encoders and in particular to an enhanced analog-to-digital encoder which employs delta modulation.
There is a wide range of commercial, industrial, scientific and military systems which are employed for high accuracy sensing of a variety of physical phenomena. Many of these systems convert the sensed analog data to digital data which may be readily transmitted and processed without degrading the dynamic range, resolution, phase or linearity of the data. In these systems the analog signals provided by the sensors (e.g., microphones, hydrophones, geophones, optical sensors, infrared sensors, image scanners, magnetic field sensors, etc.) are prepared for digitization by signal conditioning electronics including, for example, pre-amplifiers, equalizers, controlled gain circuits, anti-alias filters, sample and hold circuits, etc. The conditioned analog signals are then digitized by analog-to-digital encoders. The primary cause for limited data fidelity and inadequate system performance is the distortion and noise introduced into the data signal by the A/D encoder and by the signal conditioning electronics.
In most systems, the best fidelity achievable by current A/D encoders with their corresponding signal conditioning circuits is 90-100 dB of signal to distortion ratio (SDR). Although there is no single universally applicable measure of A/D encoder fidelity (because of the many different applications and environments in which A/D encoders are employed), the signal to distortion ratio defined below is an accurate measure for a broad range of A/D applications.
As defined, the SDR does not encompass DC offset, scale factor error, or phase delay distortion. The SDR (as defined below) encompasses the following A/D encoder errors and performance terms: non-linearity (linearity); differential non-linearity (differential linearity); harmonic distortion; intermodulation distortion; quantizing noise; all other noise (Johnson, Gaussian, etc.); clipping; dynamic range (instantaneous, two-tone); resolution; and monotonicity. Thus, it can be seen from the above list, that the signal to distortion ratio (SDR) takes into account a large number of considerations relevant in the measure of A/D encoder fidelity.
In order to determine the SDR, a maximum signal frequency F.sub.ms, which is the maximum frequency of interest in the spectrum of the sensed analog signal to which the A/D encoder will be applied, is assumed. Two sine wave tones (having tone frequencies of, for example, 71% and 83% of F.sub.ms and having equal amplitude) are combined and fed as an input test signal to the A/D encoder input. The A/D encoder output is subjected to spectral analysis and the distortion power is defined as the sum of all the A/D output power from zero frequency to F.sub.ms except for the energy right at zero frequency and that at the two frequencies in the input test signal. (Any noise which is lost in notching out zero frequency and the two test signal frequencies is estimated and added to the measured distortion power.) The sine wave equivalent signal power is defined as the square of the sum of the RMS amplitudes of the two tones in the input test signal, measured at the A/D encoder output. (This definition of sine wave equivalent signal power gives the power of a single tone test signal which would have the same peak to peak amplitude as the actual two-tone test signal.) The signal to distortion ratio (SDR) is defined as the maximum obtainable ratio, as the input signal strength is varied, of the above-defined sine wave equivalent signal power to the above-defined distortion power, and is conventionally expressed in decibels (dB). For A/D encoders which require that a sample and hold circuit precede them, the SDR is measured on the combined sample and hold circuit and A/D encoder.
There are a number of existing classes of methods for A/D encoding, including:
1. Integrate and Count Voltage-to Frequency and Count PA1 2. Successive Approximation PA1 3. Delta Modulation Delta Sigma Modulation PA1 4. Flash Conversion PA1 5. Josephson Junction Devices
The class 1 encoders (Integrate and Count and Voltage-to-Frequency and Count) are very slow and are used mainly in digital volt meters and digital multimeters where the measured analog signal is assumed to be stationary.
The class 2 encoders (Successive Approximation) cannot maintain signal to distortion ratio performance above 90-100 dB because of drift, with time and temperature, of resistor (or capacitor) ratios. These encoders have extremely high sensitivity to component ratios, and require a sample and hold circuit and an anti-alias filter to precede them in most applications. These additional circuits can add substantial distortions to the system. Despite the drawbacks of class 2 encoders, they offer the best performance of the five above-mentioned classes for a broad range of applications.
The class 3 encoders (Delta Modulation and Delta Sigma Modulation) generate data too voluminously, for example, a single integrator implementation of this class of encoder generates 20,000 bits per cycle of input signal at the maximum frequency of interest (F.sub.ms) in order to achieve 120 dB of SDR. This high output data rate relative to the true information content causes untoward difficulties in processing, storing and transmitting the data. The SDR of this class of encoder is governed by a noise floor of so-called "granular noise" (which is a form of quantizing noise). A single integrator implementation of this class of encoder gains only 9 dB of granular noise reduction, and thus gains only 9 dB of SDR, for each doubling of its output bit rate relative to the maximum frequency of interest of its input signal. Therefore, the bit rate required grows exponentially with increasing SDR. Due to this exponential bit rate growth, this class of encoder is most useful for low SDR applications. Double integration implementations of this class of encoder can achieve 15 dB of SDR growth per doubling of bit rate and are therefore useful for applications of medium SDR. However, even with double integration the output bit rate is impractically high for applications requiring 100 dB or more of SDR.
The class 4 encoders (Flash Conversion) are limited to low SDR applications (approximately 60 dB) since each additional 6 dB of SDR doubles the complexity of the flash converters. Thus, the flash converters are not suitable for high SDR (100 dB or more) applications but instead are more suitable for very high bandwidth signals.
The class 5 encoders (Josephson Junction devices) are not in present commercial use. These devices will operate on the unique quantizing properties of Josephson junctions and offer very high speed and accuracy. However, they will require a cooling system to keep them within several degrees of absolute zero, thereby making them suitable only for specialized applications.
In summary, there is a need in the art for an analog-to-digital encoder having a high signal-to-distortion ratio, having a low data rate relative to the signal frequency bandwidth of interest, and having a reduced amount of granular noise.